Associated with recent small-sized high-performance mobile phones and electronic devices, nonaqueous electrolyte batteries such as lithium secondary batteries exhibiting high energy density with high voltage draw attention and are widely used as the power source thereof.
Conventionally, it is known to use lithium titanate as an active material for a negative electrode in such lithium secondary batteries and lithium ion batteries (see, Patent Document 1). Further, it is known to substitute a part of the element of the lithium titanate with Fe (see, Patent Document 2), with Cu (see, Patent Document 3) and with other transition metals and the like (see, Patent Documents 4 to 6).
Patent Document 1: JP 06-275263-A
Patent Document 2: JP 2001-185141-A
Patent Document 3: JP 2001-250554-A
Patent Document 4: JP 2004-235144-A
Patent Document 5: JP 10-251020-A
Patent Document 6: JP 2000-156229-A
Particularly, Patent Document 4, describes an invention of “a negative electrode active material for nonaqueous secondary battery having a lithium transition metal composite oxide having a spinel structure containing an alkali metal and/or alkali earth metal.” (claim 1), and “the lithium transition metal composite oxide is a negative electrode active material for nonaqueous secondary battery according to claim 1 or 2” containing aluminum (claim 3). It also describes that “by including aluminum, a part of the transition metal of the lithium transition metal composite oxide is substituted with aluminum, thus, it is conceivable that the crystalline structure is stabilized and the cycle performance is improved.” (paragraph [0024]). However, since there is described that “by containing alkali metal and/or alkali earth metal, a crystalline structure of the lithium transition metal composite oxide having a spinel structure is stabilized, thus, it is conceivable that the cycle performance is improved.” (paragraph [0022]), it is essential that an element entering into 16d site “contains an alkali metal and/or alkali earth metal” as described in claim 1. Thus, it is not suggested that the cycle performance is improved by substituting a part of the transition metal of the lithium transition metal composite oxide only with aluminum, and moreover, it is not disclosed that a high rate charge and discharge performance is improved by such a substitution.
Patent Document 5 describes an invention of “metal-substituted lithium titanate represented by a general formula LixMyTizO4 (where M is a metal having a valence of two or more, and 0.5≦(X+Y)/z≦2), wherein a part of a lithium component of lithium titanate is substituted by a metal having a valence of two or more.” (claim 1), and aluminum is described as a metal having a valence of two or more (claim 2). However, since it describes that “doping and dedoping of lithium ions are facilitated by substituting a part of a lithium component with a metal having a valence of two or more, thereby improving the performances such as battery capacity and the like when used as an electrode for lithium battery.” (paragraph [0006], only a part of the lithium component is substituted with aluminum, and titanium component is not substituted therewith. Further, this invention is not intended to provide a lithium ion battery having an excellent high rate charge and discharge performance.
It is further known to use a spinel compound represented by a composition formula: Li4AlyTi5-yO12 (y=0, 0.10, 0.15, 0.25) as an electrode for lithium battery (see, Non-Patent Document 1).    Non-Patent Document 1: Journal of The Electrochemical Society, 152(1) A186-A190 (2005)
The spinel compound in Non-Patent Document 1 is one in which only a part of Ti of Li4Ti5O12 (lithium titanate) is substituted with Al. Although it is described that a higher capacity lithium ion battery can be obtained by using this Al-containing lithium titanate, it is not described that a lithium ion battery having an excellent high rate charge and discharge performance can be obtained. Moreover, in the above-described composition formula, Al is a representative element of trivalent (Al3+), and since only a part of tetravalent Ti component (Ti4+) is substituted with Al having the same number of moles and no monovalent Li component (Li1+) is substituted with Al, a part of Ti becomes partially pentavalent (Ti5+).
On the other hand, a spinel compound represented by a composition formula: Li[Li(1-x)/3CrxTi(5-2x)/3] is further known as a negative electrode active material for lithium ion battery (see, Non-Patent Document 2).    Non-Patent Document 2: Journal of Power Sources, 125 (2004) 242-245
The spinel compound in Non-Patent Document 2 is one in which a part of Li- and Ti components of Li4/3Ti5/3O4 (lithium titanate) is substituted with Cr. Although it is described that diffusion coefficient and high rate discharge performance are improved by using this Cr-containing lithium titanate, no substitution with any other metal than Cr is described. Moreover, as described below, when a part of Ti component of lithium titanate is substituted with Cr, the improvement of high rate discharge performance is not sufficient.
That is, in the above-described composition formula, Cr is trivalent (Cr3+) and Ti is tetravalent (Ti4+). However, since Cr is a transition metal, the valence number of Cr will vary due to electrochemical reduction of lithium titanate when such lithium titanate is used as a negative electrode active material for battery. In order for a battery including a negative electrode containing lithium titanate to have an excellent high rate discharge performance, it is important that tetravalent Ti (Ti4+) and trivalent Ti (Ti3+) coexist in lithium titanate contained in the negative electrode which has been charged for the first time after the composition of the battery and it is further desired that trivalent Ti (Ti3+) exists more than tetravalent Ti (Ti4+). However, so the valence number of Cr varies due to the discharge of the battery, trivalent Ti (Ti3+) is insufficiently produced, thereby making it difficult to exhibit an above-described effect.